1 14 IT.NITI KSKOI.A. M.-N. Kl. 



The IxMildcr iVoin uiiicli this spcciiiicn was taken has probably been 

 transported hy the laiui-iee lidin sonic of the numerous elongated norite 

 masses in the nei,i;lihc)iiih<)(i(l of 'I'vedestrand and further north in liamle, 

 known especially because they, at many points, are the mother-rocks ot 

 nickel-bearing pyrrhotite'. Such a locality, in close vicinity of Oybvag, is 

 I loias near Tvedestrand. 



A. Lacroix^ gave a microscopic-petrographic description of gabbros 

 from Odegarden in Ijnmle and I loiås (by Lacroix written Heias). He paid 

 much attention to the coionas, and found zones of garnet in different kinds 

 ol" such coi'onites. From the I loias mine he described a corona around 

 hypersthene, consisting of green hornblende with spinel, quite as that 

 observed by me. Lacroix also observed cases where the central kernel 

 consists of olivine surrounded by double zones of hypersthene and horn- 

 blende (op. cit. p. 234). The granular character of the hypersthene in my 

 slice suggests the idea that die hypersthene would here also be a synantetic 

 mineral having completely replaced the olivine. 



Among the two generations of garnet that one occui'ring as large 

 crystals no doubt crystallized directly îYom the magma. At first sight, 

 one could believe that the garnet might belong to quite an early stage 

 of rock development, an eclogitic stage. But really it is not so. The 

 distribution of the garnet-bearing varieties as boundary-zones in the rock- 

 masses, a fact with which at first professsor J. Schetelig kindly made me 

 acquainted, gives some evidence on this point. 



The relations have been described by J. H. L. Vogt^^ from the nickel- 

 ore fields of Ertelien in Ringerike, D3'rhaug in X'ærdalen etc. On a minute 

 scale the phenomenon, in pyrrhotite-bearing norite, is a tN'pical corona, 

 appearing only at the contact against plagioclase. But it may happen that 

 there is, at the boundary between the sulphide and the norite a compact 

 zone of garnet from 5 to 10 mm in thickness passing gradually over into 

 the normal norite. 



The formation of garnet is favoured by a high proportion of iron, as ■ 

 apparently the almandites have a wider field of existence than diose mixtures 

 containing appreciable amounts of magnesia. Now the iron in proportion 

 to the magnesia, in a crystallizing magma-mass, is presumably highest at 

 the contact against a segregated mass of iron sulphides. Therefore garnet 

 has been formed thei e. 



' J. H. L. Vogt, Bildung von Erzlagerstätten durch Differentiations-Processe in basischen 

 Eruptivmagmata, II. Zeitschr. f. praktische Geologie, 1893, p. 125. Older litteratur has 

 been quoted there (p. 131). 



2 A. Lacroix, Contribution à l'étude ' des gneiss à pyroxene et des roches a wernérite. 

 Bull. soc. fr. de minéralogie, XII, 1889. 



3 Op. cit. p. 140. 



